1. Field of the Invention
This invention relates to a toner used in a process by which an electrostatic latent image is converted to a visible image, in particular, a toner that can provide electrophotographic images reproduced in a high image quality and a high resolution, a one-component type developer or two-component type developer making use of such a toner, and a process for producing toner particles.
2. Related Background Art
There is an image forming method in which an electrical or magnetic latent image formed on a recording member is converted to a visible image by attracting to the latent image, electrosensitive or magnetosensitive fine particles called a toner. As electrophotography, which is a typical example thereof, a large number of methods are known in the art as disclosed, for example, in U.S. Pat. No. 2,297,691. In general, in this electrophotography, an electrostatic latent image is formed on a photosensitive member, utilizing a photoconductive material and according to various means, and subsequently the latent image is developed using the toner to form a toner image. The toner image is transferred to a transfer medium such as paper if necessary, and then the toner image thus transferred is fixed to the transfer medium by heating, pressing or using solvent vapor. A copy is thus obtained.
In recent years, people have shown interest in the improvement of image quality of electrophotographic reproductions, and hence it is sought to provide an electrophotographic process that can obtain high-grade copies with ease. Toner is formed of fine particles mainly composed of a resin and a coloring material such as a magnetic material, carbon black or a dye or pigment, which usually have a particle diameter in the range of 6 to 30 .mu.m. In the formation of electrophotographic images, various processes are used to form the images, and are known to have influence on their image quality. In general, an improvement in image characteristics, specifically, in image reproducibility such as highlight reproducibility or shadow reproducibility, can bring about an improvement in image quality of electrophotographic images. For such purpose, it is considered necessary to use a toner with a small particle diameter, what is called small sized toner, as the above toner.
Toners have been hitherto commonly obtained by mixing and melting in a thermoplastic resin a coloring material comprised of a dye or pigment and a magnetic material to uniformly disperse the coloring material, followed by pulverization and classification to produce a toner having a desired particle diameter. This method is relatively stable as a technique and can enjoy relatively easy control of the materials and processes. In this method, however, contents are laid bare at shear cross-sections, and hence low-melting components (which make a melting point low) and release components (which impart releasability) can not be incorporated in large quantities enough for them to be effective. In addition, the classification must be carried out at a severe level in order to achieve the small particle diameter, resulting in an extremely low yield and an impractical industrial application.
In recent years, methods for producing toners by polymerization are proposed as methods to overcome the above disadvantages. These are disclosed in Japanese Patent Publications No. 36-10231 and No. 51-14895 and Japanese Patent Applications Laid-open No. 53-17735, No. 53-17736 and No. 53-17737. In the methods disclosed therein, a binder resin, a colorant such as a dye or a pigment, materials that are required to be contained in a toner as exemplified by a magnetic material, carbon black, a charge control agent and a release agent such as wax or silicone oil are dissolved or dispersed in polymerizable monomers optionally together with a polymerization initiator and a dispersant to form a polymerizable composition, and this polymerizable composition is dispersed in an aqueous continuous phase containing a dispersion stabilizer, using a dispersion machine, to form a dispersion of fine particles, followed by polymerization of this dispersion to effect its solidification so that toner particles with the desired particle diameters and composition can be obtained.
The above methods certainly enable omission of the steps of pulverization and classification and are expected to be effective for energy saving, improvement in process yield and cost reduction. However, products obtained may often have a particle size distribution which is varied according to production conditions such as stirring conditions and a production scale to yield a relatively broad particle size distribution. Further, it has been difficult to make toner particles stable to have a desired particle diameter required for obtaining images reproduced in a high quality and a high resolution.